This fixes the half of bug #277, when one attempts to publish from a desk or workspace.

When `PUBLISH_RELATED_FAIL_BEHAVIOR = fail` and a related asset fails to publish, we now call `raise_conflict` to return a 409 and do a better job displaying *all* the appropriate error messages.

When `PUBLISH_RELATED_FAIL_BEHAVIOR = warn` and related asset fails to publish, we call a new moethod, `show_accepted()`. This method returns a 202 status code, which I'm abusing a bit here, but it comes closest to what we want. The story properly publishes and disappears from the desk, but a new handler in the Ajax code also shows the errors when related failed to publish. I also added code to abort at the end of the Desk `publish` callback when a request is Ajax so that no other stuff gets sent back to the browser. This is because in "warn" mode, we wnt the full request to succeed, with no rollbacks or anything, and all subsequent code should execute, so that the story will properly be published and removed from workflow.

Tomorrow I'll have to figure out what to do about non-ajax publish requests in order to properly and finally fix bug # 277.

With `PUBLISH_RELATED_FAIL_BEHAVIOR = fail`, things work fine now when publishing via "Check in and Publish" in the story profile and when selecting that option on a desk or workflow, which is an Ajax call.

One side effect is that if the publish fails because the story itself fails to publish (rather than because a related fails), the story is instantly checked out to the user again and put back into workflow. In the interrim, the asset will have been checked in, so the result is a new version number. I think that this is a very minor issue that most folks won't even notice, and is far better than what we had, where things were checked in and sometimes removed from workflow. This is better: other than the new version, it looks like the same page as before, with all the same data, but a nice status message explaining the failure.

In order to properly catch an error when there are no destinations, that exception is now thorwn as an "invalid error" exception rather than a fatal exception. That indicates that it's something to inform the user of, rather than something unexpected (which is a 500). Looking at it, I think it was silly to have been throwing a burn error for that particular error; an invalid error is a much better choice.

Still to do to finish fixing this bug:

* Fix media to follow the same pattern. * Make sure that `PUBLISH_RELATED_FAIL_BEHAVIOR = warn` works as expected * Make sure that publishing from search results works as expected * Make sure that bulk publish works as expected. * Make sure all tests continue to pass.

This time, always die on non-ajax requests. This allows the "publish later" option on the Publish desk to throw an error and return to the same page, rather than load the scheduling page and show the error there.